Can we create Business Rule on ParentCustomerId field on contact entity

We have one OOB field "ParentCustomerId" and the other custom field "PartnerAccount" on the contact entity.

The business requirement is at any point of time, we should allow user to select only one field either  ParentCustomerId/PartnerAccount.

So, we are planning to write the business rule on these fields if any one field contains data disable the other field.

However in the business rule, not able to find the field "ParentCustomerId".

Please let us know why we cannot create business rule on ParentCustomerId.

And is there any other way OOB we can achieve this requirement.